Method and Arrangement for Improved Handover by Muting Interfering Nodes

ABSTRACT

In a telecommunication system, a method for improved handover signaling includes temporarily enhancing the radio environment of a user engaged in handover signaling by muting one or more dominant interfering cells during the signaling process, and coordinating the muting with the actual handover signaling.

TECHNICAL FIELD

The present invention relates to communication systems in general,specifically to methods and arrangements for improved handover in suchsystems.

BACKGROUND

The ability to manage user mobility without service interruption is afundamental requirement in cellular networks. Particularly sensitive isthe so-called handover or handoff procedure in which a user'scommunication link(s) is transferred from one (or more) base station(s)to one (or more) other base station(s) in the middle of an activesession. The purpose of the handover procedure is to preserve ongoingcalls or sessions, when moving from one cell to another.

The handover process can be “soft”, in which case an active set ofmultiple base stations maintains simultaneous connections with a givenuser and base stations are added and removed from the active set asradio conditions change, or “hard”, in which case a single serving basestation passes on a connection in its entirety to another serving basestation.

The decision whether to perform handover or active set update is usuallymade by a network node such as the base station controller in GSM, orthe radio network controller in WCDMA. As a basis for this decision, thenode receives information about the radio link quality from the basestations and mobile units under its control. During a call, the mobileunit sends measurement results to the base station or base stations withwhich it is communicating, either periodically, or when requested to doso by the network, or whenever some other pre-defined criterion isfulfilled. The measurement results typically contain measurements of theradio signal strength and quality of the downlink (from the basestation(s) to the mobile unit) of the call, as well as the signalstrengths of a number of neighboring base stations, e.g. six neighboringbase stations in GSM. The serving base station or base stations measurethe uplink (from the mobile unit to the base station(s)) radio signalstrength and quality of the call and forward these measurement results,together with those from the mobile unit, in a measurement report to theaforementioned node that is responsible for handover and active setupdate decisions. From the information in the measurement reports, thenode is then able to decide whether a handover to another cell or anactive set update is needed.

A variation on the abovementioned procedure is that the mobile unititself initiates the handover process based on its own measurementresults. This is an option, for example, in GPRS systems.

Irrespective of which node actually makes the decision, handoversignaling is necessarily performed at or near the cell borders, far fromthe serving base station(s) in relatively poor radio conditions. Pickingthe optimal time to initiate handover or active set update can bedifficult in practice due to factors such as cell plan irregularities,neighbor cell measurement constraints, measurement filtering, hysteresisvalues, and channel management timers. As explained in more detailbelow, sub-optimal handover behavior can have a strong negative impacton service quality and overall system performance, making it essentialthat the handover procedure be robust to poor radio conditions.

State-of-the-art cellular networks typically employ a very tight reuseof radio resources to maximize spectral efficiency and simplify networkplanning. An exemplary method in GSM is Fractional Load Planning (FLP)in which frequencies are reused for traffic in each cell, so-called1-reuse. Fewer transceivers than there are available frequencies aretypically installed in each cell in order to guarantee that only afraction of these frequencies are in use at any given time irrespectiveof the amount of traffic, hence the term “fractional loading”. Users inan FLP network perform frequency hopping between all the allocatedfrequencies, but in different patterns in different cells to createstrong but sporadic interference. Any bit errors occurring during thetimes of interference can generally be corrected with the help ofchannel coding and interleaving, and the resulting spectral efficiencyclearly exceeds that of traditionally planned networks with a sparsefrequency reuse.

In traditional sparse reuse networks, system performance is relativelyinsensitive to users being connected to sub-optimal base stationsbecause the interfering cells are typically far away from both theserving cell and its neighboring cells. Mobile units drifting well intoa neighboring cell before performing handover do not generally cause asignificant increase in interference and the demands on the handoverprocess are therefore lower. In state-of-the-art tight reuse networks,however, interfering cells are close by and being connected to the mostappropriate base station is essential if the high spectral efficiencypotential is to be achieved. In many cases, the serving cell and itsneighbors will be strong interferers and drifting well into aneighboring cell can dramatically increase interference levels, therebydisrupting user services and reducing system capacity. Battery lifetimesare also negatively affected.

Soft handover solutions are generally more robust than those using hardhandover, but they involve higher complexity and cost in infrastructureand terminals, and they demand more transmission and air interfaceresources. In many practical situations, the introduction of softhandover is unfeasible.

Hard handover is easier and cheaper to implement and it utilizes fewerresources, but achieving the requisite robustness can be difficult,particularly in state-of-the-art networks, for the reasons outlinedabove.

SUMMARY

An object of the present invention is to enable an improved handoverprocedure.

A specific object is to temporarily reduce interference experienced by anode, whereby the C/I ratio is temporarily increased.

Another specific object is to temporarily reduce interference duringhandover signaling.

Another specific object is to enable coordinated muting and handoversignaling in a communication system.

These and other objects are achieved in accordance with the attachedclaims.

Briefly, the present invention aims to improve handover robustness bytemporarily enhancing the radio environment of a user engaged inhandover signaling. This is achieved by muting one or more dominantinterfering cells during part or all of the signaling process, either inall handover cases, or only when some specific conditions are fulfilled,for example, a first handover command did not reach the user, or theuser is deemed to be in sufficiently poor radio conditions according tosome criteria.

Advantages of the present invention include:

-   -   A more robust handover procedure;    -   Increased number of early successful handovers;    -   Decreased number of lost frames;    -   Decreased interference;    -   Improved service quality;    -   Fewer service interruptions such as dropped calls;    -   Increased network capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 illustrates a handover situation in which methods andarrangements according to the present invention can be applied;

FIG. 2 illustrates an embodiment of a system according to the invention;

FIG. 3 is a schematic flow diagram of embodiments of methods accordingto the invention;

FIG. 4-7 illustrates the signaling steps in an embodiment of a methodaccording to the invention;

FIG. 8 illustrates a further embodiment of a method according to theinvention;

FIG. 9 illustrates an embodiment of a system and arrangements accordingto the invention;

FIG. 10 is a diagram illustrating the relation between carrier andinterference according to prior art;

FIG. 11 is a diagram illustrating a comparison between prior art and thepresent invention.

FREQUENTLY USED ABBREVIATIONS

-   -   ACK Acknowledgement    -   AMR Adaptive Multi-Rate speech coder    -   BCCH Broadcast Control CHannel    -   BER Bit Error Rate    -   BLER Block Error Rate    -   BSC Base Station Controller    -   BS Base Station    -   BSIC Base Station Identity Code    -   C/I Carrier-to-Interference ratio    -   CS Circuit-Switched    -   CHAT CHannel Allocation Tiering    -   FER Frame Erasure Rate    -   FLP Fractional Load Planning    -   GSM Global System for Mobile communication    -   GPRS General Packet Radio Services    -   L2 Layer 2    -   L3 Layer 3    -   MSC Mobile services Switching Center    -   MU Mobile Unit    -   OFDM Orthogonal Frequency Division Multiplexing    -   PS Packet-Switched    -   RXLEV Measure of received signal strength in GSM    -   RXQUAL Measure of received quality (bit error rate) in GSM

DETAILED DESCRIPTION

By way of illustration, the present invention will be described belowprimarily in the context of inter-cell handover for circuit-switched(CS) traffic in a Global System for Mobile Communication (GSM) system.However, the use in other types of systems, for example those usingOrthogonal Frequency Division Multiplexing (OFDM), or for intra-cellhandover, or for packet-switched (PS) traffic utilizing handover or cellreselection, is also conceivable following the same general principles.For the sake of simplicity, all types of cell change during an activesession are referred to as “handover” in the text below.

Consider the situation as illustrated in FIG. 1. The system comprises aGSM mobile unit MU communicating with a serving base station BS1 in cellA. Simultaneously, the mobile unit MU is moving, traveling from the cellA of the serving base station BS1 into the cell B of a candidate basestation BS2. In the illustrated system the two base stations BS1, BS2are associated with a common base station controller BSC: However, theinvention is equally applicable to situations where the base stationsare associated with different base station controllers as well as withdifferent mobile services switching centers (MSC:s).

The mobile unit MU regularly measures, according to prior art, thereceived signal strength on frequencies used by neighboring basestations in accordance with a neighbor cell list that it receives fromthe network. During idle time slots, it also attempts to identifyneighboring base stations by decoding their Base Station Identity Code(BSIC). A correctly measured BSIC is required together with a receivedsignal strength measurement in order for the measurement to be used in ahandover decision. Once received by the network, the measurements arefiltered and handover criteria are evaluated in a network node,typically the Base Station Controller (BSC). Such handover criteria mayinclude signal strength hysteresis values to minimize ping-ponghandovers as well as other signal strength or timing penalties triggeredby previous channel management operations, for example an intra-cellhandover.

The process of neighbor cell measurement, identification, reporting,filtering and handover criteria evaluation is performed before anyhandover command can be sent. It is often carried out in an irregularcell plan with fast-moving users and an inhomogeneous radio environment.For these reasons, handover commands are usually sent to mobile unitswhen they are already in the nominal coverage area of the candidate basestation BS2 and moving further away from the serving base station BS1all the time. With the tight reuse of radio resources typically employedin state-of-the-art networks, the handover process must clearly beexecutable in poor radio conditions in order to prevent dropping calls.

In accordance with the above reasoning, a handover command will usuallybe sent to the mobile when it is already in cell B. At this point in astate-of-the-art network with a frequency reuse of one, the base stationin cell B is likely to be the dominant source of downlink interference.

According to prior art, one method of improving the robustness forhandovers is to temporarily increase the power of a serving base stationwhen a command to perform a handover is transmitted in an attempt toincrease the C/I experienced by the mobile unit to which the handovercommand is directed. However, this method spreads more interference inthe network and may not be applicable in many cases since the servingbase station is often already transmitting at maximum power before sucha handover command is to be transmitted. Moreover, the gains from themethod diminish the further into the target cell the mobile unit moves,contrary to the desired behavior of a technique aimed at increasinghandover robustness.

Instead, as identified by the inventor, in order to improve thepossibility of correct reception of a handover command it could be moreefficient to temporarily reduce the interference experienced by themobile during actual handover signaling.

Briefly, the present invention improves handover robustness bytemporarily enhancing the radio environment of a user engaged inhandover signaling. This is achieved by muting one or more dominantinterfering cells during the signaling process.

Accordingly, the present invention improves handover signalingperformance by temporarily muting one or more dominant sources ofinterference at appropriate times. Basically, the invention aims totemporarily increase the C/I experienced by a mobile performing handoverby reducing interference I rather than increasing carrier strength C,which is the standard prior art approach to improving handoverrobustness, as used for example in soft handover.

FIG. 2 represents a general system in which methods and arrangementsaccording to the invention can be implemented. The system comprises auser node e.g. mobile unit MU1 communicating with a first node e.g. basestation BS1 and experiencing interference I from at least one of aplurality of nodes e.g. base stations BS2, BS3, BS4. The first basestation BS1 experiences interference i from at least one of a pluralityof other users e.g. mobile units MU2, MU3. For downlink signaling fromthe first node to the user, the plurality of nodes are the primarysources of interference. For uplink signaling from the user MU1 to thefirst node, the primary sources of interference i are the plurality ofother mobile units. According to FIG. 2 the first and second nodes e.g.base stations BS1, BS2 are associated with a common control node e.g.base station controller BSC. However, the invention is equallyapplicable to a situation where handover is required between differentbase station controllers.

Basically, with reference to FIG. 3, an embodiment of a method accordingto the invention comprises muting S1 one or more second nodesinterfering with the communication between a first node and a user, andcoordinating S2 the muting S1 with handover signaling between the firstnode and the user.

According to a more specific embodiment, also with reference to FIG. 3,in order to enable muting a logical node issues muting commands to theinterfering nodes in such a way as to coordinate the actual muting ofthe interfering node with the handover signaling.

According to another specific embodiment of a method according to thepresent invention, the muting step S1 is preceded by an additional stepof identifying S3 one or several dominant interfering nodes that need tobe muted.

With reference to FIGS. 3-7, a specific embodiment of the invention willbe described in the context of downlink handover signaling in a GSMsystem. Once again, the model system comprises a mobile unit MU movingfrom a first cell associated with a first base station BS1 into thecoverage of a second cell associated with a second base station BS2. Thetwo base stations according to the embodiment are associated with acommon base station controller BSC, however the invention is not limitedto this case but is equally applicable to the case where the basestations are associated with separate base station controllers.

The mobile unit MU continuously performs measurements of the radio linkand the signal strengths of neighboring base stations. The first basestation BS1 continuously performs measurements of the radio link andreports these measurements along with those from the mobile unit MU tothe base station controller BSC, as indicated by the broken arrows inFIG. 4. At some point in time, the base station controller BSC decides,based on the measurements, that it is necessary to perform handover tothe second base station BS2.

With reference to FIG. 5, the base station controller BSC issues amuting command to the interfering base station BS2. Preferably, thecommand comprises information regarding what frequency or set offrequencies to mute, and for what time slot the muting should beperformed. In a coordinated fashion, the base station controller BSCissues the handover command to the first base station BS1. Meanwhile themobile is moving further into the cell of the second base station,whereby the C/I decreases i.e. the interference I increases

At a predetermined time slot, see FIG. 6, the handover command istransmitted to the mobile unit MU and at the same time, the interferingbase station BS2 temporarily mutes itself based on the muting commandfrom the base station controller.

Finally, as shown in FIG. 7, the handover is completed and thepreviously interfering node has resumed transmission, now as a firstcommunicating node and the previous first communication node is now asecond interfering node.

In one embodiment of the present invention, transmission on theappropriate frequency, or frequencies in the case of frequency hopping,and time slot from the base station in cell B is simply muted at thetime that the handover command is sent to the mobile from the basestation in cell A, thereby increasing the C/I and the likelihood thatthe handover command is received correctly. In an alternativeembodiment, the appropriate transmission from several interfering basestations could be muted in this manner.

Since there is a cost involved in temporarily muting transmissions, bothin terms of lost bursts or frames for users in the muted cells, and interms of control signaling in effecting the muting, an alternativecost-effective embodiment may be only muting transmission if certainpredetermined criteria are fulfilled. One possibility would be toconsider the measurement reports for the radio link to the serving cell,for example RXLEV and RXQUAL reports in GSM, and only apply theinterference muting if the link quality to the serving base station isbelow some pre defined threshold. Another possibility would be to onlyperform muting if an initial handover attempt fails. Subsequent attemptsare likely to take place in even tougher radio conditions due tocontinued mobility away from the serving base station making muting aneven more appealing option. Combinations of these and other criteria fortriggering cell muting are of course possible. Utilizing knowledge ofmobile position and/or velocity would be one such alternative.

A general criterion for muting an interfering cell or node could be thatthe C/I experienced by a user about to perform handover falls below apredetermined threshold. Other exemplary criteria for muting aninterfering cell or node could be that one of the bit error rate (BER),frame erasure rate (FER), or block error rate (BLER) exceeds arespective predetermined threshold value. Also, other criteria could beused.

The description so far has used handover command transmission on theserving cell downlink to illustrate the principle of the invention, butthe idea can readily be applied to other handover signaling, both on theuplink and downlink.

An example of when uplink interference muting may be beneficial is inthe case of handover command segmentation, as illustrated by the systemin FIG. 8.

In this case, the user or mobile unit MU1 must acknowledge reception ofa first segment of a handover command before a second segment can betransmitted from the first base station BS1. The handover commandacknowledgement is sent to the serving base station BS1 on the uplinkwhose quality may be poor due to the mobile unit already having passedthe nominal cell border.

For the specific case of a GSM system the handover command is a layer 3or L3 message sent from the base station controller BSC to the servingbase station BS1. The base station BS1 divides and provides the L3message in one or several layer 2 or L2 segments. Each such segment issent to the mobile unit MU1 one at a time, each transmission requiring alayer 2 acknowledgement or L2 ACK from the mobile unit MU1 before thenext segment is transmitted from the serving base station BS1. This L2ACK is critical since the mobile unit cannot do anything before theentire handover command is received. A more favorable uplinktransmission environment may be required to achieve a successfulhandover in such a situation.

In this case the at least one second interfering node is not the secondbase station BS2, but instead other mobile units MU2, MU3 communicatingwith the first base station BS1. Consequently, it is one or severalmobile units that need to be muted according to an embodiment of theinvention in order to enable reliable handover signaling in the system.

Cell muting on the uplink, as described above, requires a fast signalingcapability in order to mute the appropriate interfering mobiles at theappropriate time.

Arrangements according to the invention will be described with referenceto FIG. 9.

With reference to FIG. 9, a system 1 according to the present inventioncomprises means 41 for issuing a muting command to a node 30 interferingwith communication between a first node 20 and a user 10, means 42 forcoordinating muting and handover signaling, and means 31 for receivingthe muting command, and means 32 for muting the interfering node 30based on the received muting command.

The system can, according to a further embodiment, comprise means foridentifying which interfering node or nodes to mute.

With reference to FIG. 9, a control node 40 according to an embodimentof the present invention comprises the means 41 for issuing mutingcommands to interfering nodes and the means for coordinating 42 handoversignaling with the muting commands.

With reference to FIG. 9, an interfering node 30 according to anembodiment of the present invention comprises the means 31 for receivingmuting commands and the means 32 for muting in response to the receivedmuting command. The node 30 can be either one of a base stationinterfering with transmissions to a mobile unit, or a mobile unitinterfering with transmissions to a base station.

The arrangement and location of the different means of the embodiment inFIG. 9 serves only as illustration and should not be viewed as limitingthe scope of the invention.

FIG. 10 illustrates an exemplary diagram of how the carrier strength Cand the interference I vary with the distance from the respective basestations in cell A and cell B in FIG. 1. The distance between the basestations is assumed, for exemplary reasons, to be 5000 m and the nominalcell border is located at 2500 m where C/I=1. Okumura-Hata's propagationmodel [1] is used for calculating the diagram.

FIG. 11 is a diagram illustrating a comparison between prior-art and theinvention. The diagram shows how C/(I+N) for a mobile station varieswith the distance from the serving base station in cell A, whereC=carrier strength, I=interference, and N=noise (−115 dBm for thiscase). The solid line represents the standard case with C and Iaccording to FIG. 10. The dashed line represents the known prior-arttechnique of the base station in cell A (temporarily) increasing itspower to improve C/I. The example illustrates a power increase or powerboost of 3 dB. The dash-dotted line illustrates cell muting according tothe invention, where the base station in cell B is (temporarily) silentor muted. For a practical implementation, the applied muting wouldappear as a peak in the C/I curve in combination with receiving thehandover command.

As is implied by FIG. 11 the effect of muting the interfering cell(s) isstrong, which opens up the possibility of further reducing the risk oflost frames for mobile units utilizing the interfering cell. Accordingto another embodiment, it is possible to only reduce the transmissionpower in the interfering cell(s) not completely mute it. Thereby,mobiles communicating with the base station will only experience areduction in received power for the affected bursts or frames;consequently, the overall quality for the affected mobiles is increasedcompared to completely muting one or more bursts or frames.

Another useful implementation of the invention is for systems utilizinga so-called CHAT-configuration [2]. In a CHAT scenario, moretransceivers than hopping frequencies are available in a cell and thesetransceivers are split into separate logical groups termed “channeltiers”. Each channel tier has its own frequency hopping sequence sointerference between the tiers is strong but sporadic, just as it isbetween cells in a standard FLP network. An intra-cell handover in suchan environment, for example to switch from a half-rate speech channel toa full-rate speech channel due to mobility away from the base station,may be essential to maintain service quality. With strong interferencetransmitted from the other tier, it may be necessary to temporarily mutethe interfering tier as radio conditions deteriorate for the user inorder to complete the intra-cell handover successfully.

Although muting will typically lead to some lost bursts or frames, theoverall impact on the network will be positive, particularly with anadaptive solution. More handovers will be successful earlier leading toreduced interference and fewer service interruptions, e.g. droppedcalls. Moreover, since frames are typically stolen each time handoversignaling is transmitted in any case, more robust handovers with cellmuting are likely to decrease the total number of lost frames in thenetwork. In many cases with frequency hopping or similar techniques, alost burst is manageable with channel coding anyway and there will be nonegative quality impact at all.

Unlike soft handover and other prior art techniques, cell muting makesthe handover procedure more robust without spreading extra interferencein the network. Moreover, with a downlink implementation, there is noterminal impact and the only extra information that needs to bedistributed in the network is a trigger for muting and the radioresource allocation of the user involved in the handover process. Cellmuting is readily applicable to both CS and PS traffic in the world'smost widespread cellular technology, GSM, as well as to other systemssuch as those based on Orthogonal Frequency Division Multiplexing(OFDM). It is applicable to both inter-cell and intra-cell handovers onboth the uplink and the downlink.

At the present time, handover performance is a major limiting factor forthe quality and spectral efficiency of state-of-the-art GSM networks. Byimproving handover performance, the methods and arrangements accordingto the present invention comprise the following advantages to GSM andother cellular systems:

-   -   A more robust handover procedure;    -   Increased number of early successful handovers;    -   Decreased number of lost frames;    -   Decreased interference;    -   Improved service quality;    -   Fewer service interruptions such as dropped calls;    -   Increased network capacity.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

REFERENCES

-   [1] M. Hata, “Empirical Formula for Propagation Loss in Land Mobile    Radio Services”, IEEE Transactions on Vehicular Technology, vol. 29,    pp. 317-325, August 1980.-   [2] “Fractional reuse through channel allocation tiering”, Patent    application number: PCT/SE01/01299.

1. A method for improved handover in a communication system, said systemcomprising a user communicating with at least one first nodecharacterized by the steps of: temporarily muting at least one secondnode interfering with the communication, and coordinating the temporarymuting of said interfering node with handover signaling between saiduser and said first node to temporarily reduce interference during atleast part of the handover signaling, thereby enabling reliabletransmission of handover signals.
 2. The method according to claim 1,characterized by muting at least one interfering base station duringtransmission of a handover command from a serving base station to amobile unit.
 3. The method according to claim 1, characterized by mutingat least one interfering mobile unit during transmission of a handovercommand acknowledgement from a mobile unit to a serving base station. 4.The method according to claim 1, characterized by muting at least onedominating interfering node.
 5. The method according to claim 1,characterized by the further step of identifying said at least oneinterfering node.
 6. The method according to claim 1, characterized bycompletely muting the at least one interfering node.
 7. The methodaccording to claim 1, characterized by partly muting the at least oneinterfering node.
 8. The method according to claim 1, characterized byperforming said muting step for all handovers.
 9. The method accordingto claim 1, characterized by performing said muting step for handoversaccording to a predetermined criterion.
 10. The method according toclaim 9, characterized by muting based on measurement reports for theradio link between the user and said first node.
 11. The methodaccording to claim 9, characterized by muting said interfering node ifthe radio link quality between said first node and said user is below apredetermined threshold.
 12. The method according to claim 9,characterized by muting said interfering node if a previous handoversignaling attempt has failed.
 13. The method according to claim 9,characterized by muting said interfering node if a carrier tointerference ratio (C/I) for the user falls below a predeterminedthreshold.
 14. The method according to claim 9, characterized by mutingsaid interfering node if a bit error rate for the user exceeds apredetermined threshold.
 15. The method according to claim 9,characterized by muting said interfering node if a frame erasure ratefor the user exceeds a predetermined threshold.
 16. The method accordingto claim 9, characterized by muting said interfering node if a blockerror rate for the user exceeds a predetermined threshold.
 17. Themethod according to claim 1, characterized by said muting step furthercomprising issuing a muting command to said interfering node; and saidinterfering node muting its transmission in response to receiving themuting command.
 18. The method according to claim 17, characterized byissuing said muting command from a control node in the system.
 19. Themethod according to claim 18, characterized in that said control node isa base station controller.
 20. The method according to claim 17,characterized by said muting command comprising at least timinginformation to coordinate the muting with handover signaling, andfrequency information to inform said interfering node which frequenciesto mute.
 21. The method according to claim 1, characterized in that saidcommunication system is a GSM network employing fractional load planningand 1-reuse.
 22. The method according to claim 1, characterized byperforming said muting and coordinating for one of inter-cell handoversignaling and intra-cell handover signaling.
 23. The method according toclaim 1, characterized by performing said muting and said coordinatingfor one of packet switched or circuit switched traffic.
 24. A systemenabling improved handover in a communication system, said systemcomprising at least one user communicating with at least a first node,characterized by: means for temporarily muting at least one second nodeinterfering with the communication; means for coordinating the muting ofsaid interfering node with handover signaling between said user and saidfirst node to temporarily reduce the interference during at least partof the handover signaling, thereby enabling reliable transmission ofhandover signals.
 25. The system according to claim 24, characterized byfurther means for issuing a muting command to said interfering node, andmeans for receiving said muting command and muting said interfering nodein response to said muting command.
 26. A node enabling improvedhandover in a communication system, said system comprising at least oneuser communicating with a node, characterized by: means for temporarilymuting a node interfering with the communication.
 27. The node accordingto claim 26, characterized by means for receiving a muting command, andsaid muting means being arranged to mute the node based on the receivedmuting command.
 28. The node according to claim 26, characterized inthat said node is either a mobile unit or a base station.
 29. A controlnode enabling improved handover in a communication system, said systemcomprising at least one user communicating with a first nodecharacterized by: means for issuing muting commands to at least one nodeinterfering with the communication to temporarily mute said interferingnode; and means for coordinating said muting commands with handoversignaling between said first node and said user to temporarily reduceinterference and enable reliable handover signaling.
 30. The nodeaccording to claim 29, characterized in that said control node is a basestation controller.